In an evaluation of x-ray images, only a relatively small region of the image may be relevant. The evaluation of the x-ray images is however greatly facilitated if surrounding regions may also be seen on the x-ray image. It is therefore known to use so-called region-of-interest filters, which allow the x-radiation to pass unhindered in a region of interest and attenuate the x-radiation in regions adjacent to this region. An adaptation of the brightness and an optional filtering in the course of the evaluation make it possible to display the region of interest with high resolution and little noise, while the regions adjacent thereto are displayed with lower quality, which however is sufficient to provide a context for the region of interest.
In the examination of objects with x-radiation, (e.g., in the case of medical use), it is desirable to reduce the radiation exposure of the examination object and nevertheless achieve a required image quality. It is known for this purpose to control a radiation dose emitted onto the examination object in dependence on a previously acquired x-ray image. Control may take place, for example, in dependence on an image brightness, a contrast, or a signal-to-noise ratio. If such a method is applied when using the filters described above, it may be problematic that the image quality in the region of the image that is covered by the filter section is reduced, as a result of which the dose control is adversely affected, which may lead to an unnecessary increase in the dose and/or to an overexposure in the transillumination section. Moreover, the transmission behavior of the filter material often differs significantly from the transmission behavior of the material of the examination object. In particular, when there is a movement of the filter for displacing a region of interest, this may lead to control oscillations or to other instabilities of the dose control. This may on the one hand increase the radiation exposure of the examination object and on the other hand reduce the image quality of transillumination images.